Actually, "who decides?" is an interesting question. What R. Emmett Tyrell in The American Spectator likes to call "the kultursmog" seems to be self-organizing. No one in particular seems to have decided that the contrafactual belief that you are the opposite sex from that easily observable in your outward morphology and with microscopic examination observable in your genetics is to be treated differently from all other delusions, not treated as a sign of mental illness, but coddled by the rest of society pretending your delusion is true, supported with hormone injections and surgery to turn you into a simulacrum of what you purport to be, but are not.